Ask about random number

[ooosawaddee3]

I want to generate random number between 1- 400,000 which is not the same value.

Example.
If X1 = 254899
X2 not have the same value as X1.
and X3 not have the same value as X1 and X2 .

I want to generate data X1 - X400,000 which not have the same value and each data have value between 1-400,000

If you know how to do that, please tell me. Thank you.

[corcor]

maybe this helps:
1) create an array (size 400000) and fill it with values from 1 - 400000

2) create a randum number X between 1 and actual array-size

3) return value of array[X] as "real random number"

4) remove the X'th entry from the array

5) continue with 2)

=> you get each value exactly once
=> you have no overhead in finding a "unused" number

[mamotron]

There's also a way to do it without using variable sized arrays.

1) Create an array of 400,000, say it's called x.

2) For each array value x[i] assign it i +1, x[0] =1, x[1] = 2, x[2] = 3, .... x[399,999]=400,000

3) For each number between 0, and 399,999, pick a random number j (you can use rand() which returns a random double between 0 and 1 and multiply rand, by 400,000)

x[i] swap x[j] (in code: a= x[i]; b=x[j], x[i] = b; x[j]=a

4) At this point you'll have a randomly sorted array that has values from 1 -> 400,000. To pick a random number, return x[k], where k goes from 0->399,999 and is increased after a new random number has been returned.

[corcor]

you are right mamotron:
resizing the array is bad, but we also can take my solution and swap x[i] with the last unused element instead of deleting it